How can I acquire more opulent homes?

In Civilization III, if you're doing particularly well, your people will sometimes voluntarily suggest that you add an upgrade to your Monarchical/Despotic/Presidential/Chancellor's/Comrade's Palace.

You can go from a dinky cave on a hilltop to a grandly posh domicile such as this:

What triggers the ability to upgrade your palace?

Solution 1:

There's a CivFanatics thread about it in which they conclude... nothing. Most people seem to think that it is somewhat like modern day achievements, triggered by certain events like building a wonder or winning a battle. Others claim it is solely score based, although that seems unlikely since it would be so easy to prove if it were true. And it seems that there is some randomness associated with it as well. Maybe the triggering event makes it so that you have a % chance to get the upgrade each turn, since some people report having received multiple upgrades in the same turn.

In the end, it seems that this is one mechanic that has never been fully understood.